For words that have only three letters, there is no 4th letter. One example is with the word “insert”, there won't be any additional letters afterwards and so has to be the word "add". The word “addict” is inside the BIP39 wordlist but you might have made use of “addi” https://bookmarkdistrict.com/story18565653/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator